How can i fix my temperature Gauge in my Vauxhall Corsa 98?

I have a vauxhall corsa 3dr and the temp guage does not move but the heat inside the car works i have been told that this could be the temp sender unit but i am unsure as to where this is and where i can possibly purchase one the car heats up fine but the temp guage stays at 0 degrees and normally i could drive maybe 30 miles and it would then be warmed up now i drive approx 80 miles and it doesnt move a fraction but once it hits approx 100 miles it moves the weather isnt cold please help